The present invention relates to the hydrometallurgical extraction of metals, particularly vanadium, niobium, tantalum and zirconium, especially from ores, mineral concentrates and from certain industrial process residues in all of which these metals are associated with other metals. In some cases these other metals constitute undesirable impurities such as aluminum, silicon and iron and in others they constitute commercially valuable materials which are extracted along with vanadium or subsequently to the extraction of vanadium as, for example, in carnotite ores from which vanadium and uranium are co-extracted. In this application "industrial process residues" or simply "residues" refers to material remaining after extraction of a primary product in a chemical or hydrometallurgical process. Such residues may be treated as waste products of the process or they may be further extracted to recover one or more constituents of commercial value.
The present invention is especially, but not exclusively, applicable to the extraction of vanadium from residues obtained in the manufacture of titanium dioxide by the high temperature fluidized bed chlorination (so-called "chloride" processing) of titanium-containing ores such as rutile, ilmenite or leucoxene or ore preconcentrates such as so-called "upgraded ilmenite" or mixtures thereof. Since these ores and ore preconcentrates contain, in addition to titanium, several other elements such as vanadium, zirconium, niobium, tantalum, chromium, iron, aluminum and silicon, the presence of which would be undesirable in the titanium dioxide which is the primary product of the process, they have to be separated and are removed from the plant as residues. These residues also contain a comparatively small proportion of the titanium extracted in the process. Examples of such residues arising from the chlorination of rutile to produce titanium tetrachloride are given in U.S. Bureau of Mines Report of Investigation Nos. 7221 (1969) and 7671 (1972). The major components of the residue described are titanium as unreacted rutile and unrecovered titanium tetrachloride, carbon (coke) added in the chlorination process, and chlorine as metal chlorides. The impurity metals -- vanadium, zirconium, niobium, iron, etc., are present in concentrated form in the residue remaining after distilling off the bulk of the titanium tetrachloride. A typical analysis of the residues appears in the following table:
TABLE I ______________________________________ Percent By Wt. ______________________________________ V 4.4 Nb 2.4 Ta 1.1 Zr 2.4 Ti 10.5 Fe 4.2 Cr 1.0 Mn .08 Al 2.2 Cl 26.0 C 33.0 SiO.sub.2 4.5 ______________________________________
It will be noted that the remainder of such residues is combined oxygen and minor amounts of other metals.
The recovery of vanadium and niobium from such residues is a commercially desirable objective. Moreover, such residues are difficult to dispose of as waste material since they contain readily hydrolyzable chlorides which generate hydrochloric acid fumes on contact with moisture and also they contain toxic metals, notably vanadium. Thus the so-called "fuming" residues cannot conveniently be stored or dumped in the state in which they are obtained from the extraction process.
In U.S. Pat. No. 3,975,495 to Bowerman a process is described for recovering niobium and vanadium from a solution obtained by aqueous extraction of vanadiferous residues similar to those used in the process of the present invention. The process differs from that comprising the present invention in that it is necessary to separate the solution containing substantially all the vanadium, niobium and zirconium from the insoluble matter consisting of carbon and unreacted titaniferous ore and subsequently to effect a separation of niobium and zirconium by boiling in presence of sulfuric acid in order to precipitate these metals while leaving vanadium in solution, thus requiring a second liquid-solid separation step. In U.S. Pat. No. 3,975,495 it is shown that if the precipitation of niobium and zirconium is attempted without the addition of sulfuric acid, substantial proportions of the niobium and zirconium contents remain in the solution along with vanadium. In the process of the present invention, in contrast, virtually complete recovery of vanadium in solution is achieved substantially free from niobium and zirconium in a single process operation not requiring the addition of sulfuric acid.
As mentioned in British Patent Application No. 16102/75, residues from the chlorination of titaniferous ores can be neutralized with lime and water to make them suitable for transport and storage; the metal chlorides and hydroxychlorides present are then converted to water-insoluble hydroxides or hydrous oxides and the calcium chloride formed dissolves in the water. However in this neutralization it is difficult to avoid use of excess of lime over that stoichiometrically required for neutralization and the resulting solid product invariably contains calcium. The presence of calcium interferes with removal of vanadium from the residues.
An alternative way of rendering such residues nonfuming, known in the art, is simply to moisten such residues with a relatively small amount of water; for example, by spraying them with from 1% to about 10% weight of water, whereby the treated residue is converted to a nonfuming, free-flowing powder which is readily stored and transported for disposal.